1. Field of the Invention
The present invention relates to a color image forming apparatus such as a color printer, color copying machine, color facsimile apparatus, or the like, which has a plurality of image forming portions using a plurality of laser scanner optical systems, drives a laser element by a pulse-width modulation signal, and draws an image by light emitted from the laser element.
2. Description of Related Art
FIG. 12 is a schematic block diagram showing the arrangement of a portion associated with laser pulse width adjustment of a conventional color image forming apparatus. The image forming apparatus reproduces tones of an image by pulse-width modulation.
An image data generating portion 101 draws an image on, e.g., a page memory on the basis of data supplied from an external apparatus (e.g., a host computer) to generate raster data in a print process, and to generate image data for adjustment upon adjusting the pulse width.
A pulse width modulating portion 102 pulse-width modulates 8-bit image data (density value) supplied from the image data generating portion 101 to generate a pulse-width modulation signal (pulse-width modulated signal). Note that the pulse width modulating portion 102 has a minimum pulse width setting portion 103 for setting a minimum pulse width of the pulse-width modulation signal generated by the pulse width modulating portion 102, and a maximum pulse width setting portion 104 for setting a maximum pulse width of the pulse-width modulation signal.
A laser driver 106 drives a laser diode 107a of a light-emitting portion 107 on the basis of the pulse-width modulation signal supplied from the pulse width modulating portion 102, thereby forming an electrostatic latent image on a photosensitive drum (not shown). Note that the light-emitting portion 107 has a photodiode 107b used to detect the light amount of a beam emitted by the laser diode 107a as an electrical signal. A current generated by the photodiode 107b increases nearly in proportion to an increase in light amount of light emitted by the laser diode 107a. Since the photodiode 107b has a low response speed, when the laser diode 107a repeats emission and putting out of light on the basis of the pulse-width modulation signal, the photodiode 107b generates a current according to the ratio between the light emitting period of time and the light putting out period of time. That is, the light emitting period of time increases with increasing value of image data, and a current generated by the photodiode 107b increases. Conversely, the light emitting period of time decreases with decreasing value of image data, and a current generated by the photodiode 107b decreases.
The current generated by the photodiode 107b is converted by a resistor 108 into a voltage value, which is buffered and amplified by an OP amplifier 109. The amplified value is then supplied to a controlling portion 105. The controlling portion 105 has a CPU 105a and an A/D converter 105b. The A/D converter 105b converts an analog signal (light amount detection signal) input from the OP amplifier 109 into a digital signal. Based on that digital signal, the CPU 105a adjusts the setting values of the minimum and maximum pulse width setting portions 103 and 104 so that the light amount of a beam emitted by the laser diode 107a matches a target value.
FIG. 13 shows the relationship between the pulse width of a drive signal (pulse-width modulation signal) of the laser diode 107a, and the light amount of a beam emitted by the laser diode 107a. As the characteristics of the laser diode, the pulse width of a beam emitted by the laser diode becomes smaller than that of the drive signal. This is because laser oscillation occurs when the quantity of carriers inside the laser diode has exceeded a predetermined value due to a current supplied to the laser diode, and a certain time period is required until the quantity of carriers inside the laser diode exceeds the predetermined value. Such delay of laser emission is called an emission delay of laser, and is a cause of the pulse width reduction of the laser beam.
In order to obtain an emission waveform with an appropriate pulse width, the pulse width of the drive signal of the laser diode 107a must be set to be larger than that of a desired emission waveform. This adjustment is called pulse width adjustment. For this reason, when the laser diode 107a is driven based on 8-bit image data (i.e., 00(hex) to FF(hex)), the pulse width output from the pulse width modulating portion 102 is saturated at a value smaller than FF(hex). On the other hand, the light amount of light emitted by the laser diode 107a increases abruptly and is saturated while the pulse of the pulse-width modulation signal reaches saturation.
In the aforementioned arrangement example, a method of inputting image data 00 (hex), and adjusting the setting value of the minimum pulse width setting portion 103 based on the light amount detection value corresponding to that data is available. However, in this case, since the minimum pulse width is adjusted by detecting a very small light amount, it is difficult to stably detect a light amount, and it is also difficult to appropriately adjust the minimum pulse width. In the above arrangement example, since the light amount has already been saturated in image data less than FF (hex), linearity of the light amount is impaired. On the other hand, a method of inputting image data FF (hex), and adjusting the setting value of the maximum pulse width setting portion 104 based on the light amount detection value corresponding to that data is available. However, in this case, since the light amount has already been saturated in image data less than FF (hex), linearity of the light amount is impaired.
To solve this problem, Japanese Patent Application Laid-Open No. 10-272801 proposed a method of pulse-width modulating, for example, image data 10(hex) near the minimum value 00(hex) of the image data to drive a light-emitting element, adjusting the minimum pulse width based on the light amount, pulse-width modulating, for example, image data F0(hex) near the maximum value FF(hex) of the image data to drive the light-emitting element, and adjusting the maximum pulse width based on the light amount.
In an electrophotographic color image forming apparatus, various methods of sequentially transferring different color images onto a recording medium retained on a conveying belt or onto an intermediate transfer belt by providing a plurality of image forming portions to attain a high-speed process have been proposed.
However, in the case of the conventional arrangement mentioned above, the following problem is posed. When an image forming apparatus has a plurality of image forming portions (e.g., four image forming portions), and has only one A/D converter for detecting the light amount, analog signals (light amount detection signals) of respective colors must be input to the A/D converter while switching them using a multiplexer. For this reason, light amount detection requires a time corresponding to the number of image forming portions, resulting in that it is time-consuming to adjust the laser pulse width.
On the other hand, when the image forming apparatus has A/D converters in correspondence with the number of image forming portions, the time required to detect the light amounts is the same as that of an image forming apparatus having only one image forming portion. However, the cost increases due to a plurality of A/D converters.